hello Jim,
You can definitely do that (no need to create style from scratch). Select desired style and open Style Creator (in MENU 2). Once you're inside, select variation you wish to modify (MAIN A, MAIN B,..) and the select either RHY2 or RHY1 (that's for drums). After you have done that, select EDIT tab and here's where you can make drum changes in style. After changes are made, you need to save your work, of course.
I highly recommend that you carefully read
PSR-S970 Reference Manual (page 30) first!
It can take some time until you get the idea what's all about, but be patient. And keep in mind: try and see what happens -you cannot damage anything.
Bogdan